1. Field of the Invention
The present invention relates to motor speed control devices and more particularly to a motor speed control device which functions stably at low voltage source.
2. Description of Prior Art
Electronic apparatuses such as a tape recorder are apt to be subminiaturized in recent years. This permits use of the driving power source of low voltage.
Following example is a motor speed control device in prior art which is used for a tape recorder and operated by low voltage. FIG. 1 shows a block diagram of the speed control device in prior art, and FIG. 2 shows a time chart of the device in FIG. 1. The speed control device comprises a tachometer generator 2 connected directly to output shaft of a motor 1 and generating alternating voltage with frequency proportional to the revolution speed of the motor 1 as shown in FIG. 2(a). The alternating voltage is transformed into square wave as shown in FIG. 2(b) by a wave shaping circuit 3 and then supplied to a trigger pulse generator 4. Trigger signal is generated corresponding to leading edge (or trailing edge) of the square wave as shown in FIG. 2(c), and signal with prescribed time width T.tau. as shown in FIG. 2(d) is generated by a timer circuit 5 in synchronization with the trigger signal. The signal with time width T.tau. is supplied to an integral control circuit 6 thereby charge or discharge state of a capacitor 7 is controlled as shown in FIG. 2(e). The output produced by charging or discharging the capacitor 7 is passed through a low-pass filter and becomes the frequency/voltage tranducing output, which is amplified by a buffer circuit 8 into sufficient power to drive the motor 1. The revolution speed of the motor 1 is controlled by the buffer output and held constant using negative feedback in the control system.
The integral control circuit 6 as principal part of the control system is specifically constituted as shown in FIG. 3. In the integral control circuit 6, transistors Q.sub.1, Q.sub.7, Q.sub.6 constitute a current mirror circuit. A constant-current circuit IS is connected to collector of the transistor Q.sub.1, and current flows through the transistor Q.sub.6 at a specified ratio with respect to that in the constant-current circuit IS. A transistor Q.sub.1 serves to control base current of the transistors Q.sub.1, Q.sub.6, Q.sub.7. Another current mirror circuit is constituted by transistors Q.sub.4, Q.sub.3. The transistors Q.sub.3 and Q.sub.4 are connected respectively to collectors of the transistors Q.sub.7 and Q.sub.6. Thereby current flows through the transistor Q.sub.4 also at a specified ratio with respect to that in the constant-current circuit IS. A transistor Q.sub.5 is connected between base and emitter of the transistor Q.sub.3, and the timer circuit 5 is connected to base of the transistor Q.sub.5. In FIG. 3, Vcc designates the DC power source and C.sub.1 corresponds to the capacitor 7.
In the above mentioned constitution, if the transistor Q.sub.5 turns ON or OFF corresponding to high or low level of output of the timer circuit 5, bias of the transistor Q.sub.3 turns ON or OFF thereby current in the transistor Q.sub.4 is switched. Charging or discharging current of the capacitor C.sub.1 is changed, and suitable selection of the charging/discharging current ratio enables gain of the control system to vary.
In conventional manner, monostable multivibrator is commonly used in the timer circuit 5 to provide signal for changing charge or discharge state. Referring to FIG. 1 again, if the revolution speed of the motor rises rapidly and the pulse generating interval by the trigger pulse generator 4 becomes less than the operating time of the multivibrator (T.sub..tau. shown in FIG. 2(d)) and a plurality of trigger pulses are supplied in the time interval T.sub..tau., the timer circuit 5 ignores all of the trigger pulses thereby charge or discharge state of the capacitor 7 does not respond to the speed of the motor 1. This may cause the speed control of the motor 1 to be unstable.
In order to eliminate such unstable control, use of two time-constant circuits or digital processing has been proposed. However, every proposal requires increase of outer parts and circuit elements therefore the apparatus becomes expensive and large in size. Accordingly, conventional manner has defect in obstructing needs to miniaturize apparatuses.